Scalar Spin Chirality from Dissipative Pumping and Lamb Shift Precession

How can a system with a real microscopic Hamiltonian, lacking explicit magnetic flux or complex tunneling amplitudes, generate and reverse circulating currents and scalar spin chirality? The authors investigate this phenomenon in a triangular triple quantum dot, where previous studies observed bias-induced current reversals but lacked a clear physical explanation for the underlying mechanism.

Using a combination of a channel-restricted second-order master equation and hierarchical equations of motion (HEOM), the authors model the triangular triple quantum dot as an open quantum system. They identify the ground-state manifold as a chiral orbital doublet and derive an exact identity linking the circulating current to the scalar spin chirality. By analyzing the dissipative kernel and the Lamb shift—a matrix-valued energy shift arising from virtual charge fluctuations—they map the system's dynamics onto an orbital Hanle effect, where an in-plane pseudospin is pumped by tunneling and rotated by a precessional torque.

The study demonstrates that the circulating current is not a direct consequence of the lead current but rather an emergent property of the orbital pseudospin dynamics. The dissipative tunneling process acts as a source that polarizes the orbital pseudospin in the plane of the triangular plaquette. Simultaneously, virtual charge fluctuations generate a Lamb field that acts as an effective magnetic field, exerting a torque that rotates this polarization into the axial direction, thereby generating scalar spin chirality and a circulating current. Because the relative orientation of the pump and the Lamb field changes with the applied bias, the system undergoes repeated current reversals as these vectors align or anti-align. The authors confirm these findings using HEOM, showing that the low-bias behavior persists beyond weak-coupling approximations and that the short-time quadratic growth of chirality provides a distinct dynamical signature of the torque mechanism.